Bug 1586050 - Don't blow away progress listeners when switching between local and remote processes, r=kmag

Whenever we do a process switch, the <browser> gets a new WebProgress instance.
When that happens, we currently handle transferring listeners from one
RemoteWebProgress instance to another, but not from a local to remote, or vice
versa. This patch changes that by tracking listeners added to the <browser> and
manually re-adding them after a process switch occurs.

Differential Revision: https://phabricator.services.mozilla.com/D48233

--HG--
extra : moz-landing-system : lando